As Rick says, the original heater isn't used, but the controls ARE used.. (modified). it's the user's option whether to retain the original factory vents (or not). Most do not, but I like using vents when feasible, so I retained them in my '56 Nomad, and plan to do the same in my '57...

The defroster ducts are very cheap on the vintage air system and if I was doing it again I would look and see if the factory duct can be modified to work. By the way I had to cut mine to get it out of the car, clutch pedal assembly in the way